The measurement of temperature is important in many applications, including, for example, manufacturing, medicine, food processing, and food storage. The most common electrical measurement devices include thermocouples, and negative temperature coefficient (NTC) thermistors.
A NTC thermistor is a thermally sensitive device whose resistance decreases with an increase in temperature. To measure a thermistor's resistance, a small current is passed through the thermistor, and the voltage drop produced is measured. Currently, NTC thermistors used for high temperature measurements (measurements in excess of −30° C.) are made of metal oxide ceramics.
The change in resistance exhibited by metal oxide ceramic NTC thermistors is strongly non-linear, and most such thermistors have a useful temperature span of up to about 300° C. The field of use of these thermistors is naturally limited to applications where temperatures do not exceed 300° C. Furthermore, metal oxide ceramic NTC thermistors are moisture sensitive, and therefore must be protected from their external environment by encapsulating them in, for example, metal, epoxy, or glass. Such encapsulation separates the thermistor from the environment to be measured, which causes a delay in response time due to a temperature gradient between the environment and the thermistor. Where a metal encapsulation is used, such as with metal tubes, the delay in response time can be reduced. However, metal encapsulation does not function well in radio frequency (RF) fields.